Patent GB-827725-A (GB)
Title: Improvements in or relating to magnetic resonance apparatus
IPC: G01R33/44
Filed: 1955-12-28
Abstract: 827,725. Electric tests. SCHLUMBERGER WELL SURVEYING CORPORATION. Dec. 28,1955 [Dec. 30, 1954], No. 37166/55. Class 37. In apparatus for detecting gyromagnetic nuclear resonance phenomena in which the macroscopic moments are initially aligned with a substantially uniform unidirectional field at right angles to which an alternating field at the resonance precession frequency is established, the moments associated with nuclei not in the magnetic field are prevented from fanning out by applying at right angles to the first alternating field, a second alternating field which, at the instant of application, is displaced 90 degrees with respect to the phase relation established by the first alternating field. As shown (Fig. 2) the paramagnetic material 10 under test is inserted in the unidirectional field produced by magnet 11 and a field alternating at the resonance precession frequency is applied to coil 12 from oscillator 18 through gated amplifier 19 and switch 22, both controlled by a timing circuit 20. After a period sufficient to allow the macroscopic moment to nutate through 90 degrees timing circuit 20 switches the oscillator output from coil 12 to coil 13 at right angles and at the same time opens amplifier 27 to connect detector coils 17 to meter 26 via detector 28. The effect of the field applied by coil 13 is to produce a field rotating in step with the macroscopic moment and to exert a restoring torque on moments which tend to move out of line with the rotating field. After this field has been applied to a time less than the relaxation time the system is turned off and the process repeated. The interval for which the apparatus is turned off may be shortened by counter-nutating the moments back to the z axis by applying a phase-reversed pulse to coil 12. Fig. 4 shows an alternative embodiment in which instead of applying a field at 90 degrees physically to the original alternating field the original field undergoes a phase change of 90 degrees. The unidirectional field is produced by a permanent or electromagnet 56 and is intensity modulated by coil 57. The alternating field is applied by coil 52 and the pick-up is at coil 64. In operation, the output of an oscillator 35 is fed through phase-splitting transformer 37 to three switches 39, 41, 43. The switches are normally open but when closed allow respectively an alternating current, the same current phase shifted by 90 degrees, and the same current phase reversed to energize coil 52. The switches are opened by multivibrators in unit 60 at appropriate times. Referring also to Fig. 5, frequency divider 61 produces pulse 82 which triggers multivibrator 58 to produce successive negative and positive pulses Tgl, Tg2 (Fig. 5 (C)). These pulses modulate the intensity of the unidirectional field (Fig. 5 (D)). The upper field strength corresponds to the frequency energizing coil 52 to produce resonance while the lower is substantially different. Pulses 83, 84 also trigger multivibrator 63 producing delay pulses 87 of duration Td (Fig. 5 (E)). The end of each pulse 87 triggers multivibrator 44, Fig. 4, to produce pulse 88 of duration t1 (Fig. 5), pulses 88 close switch 39 and allow R.F. current to produce an alternating field in sample 55. This field persists for a period sufficient to produce 90 degrees nutation in the sample. The end of each pulse 88 triggers multivibrator 45 to produce pulse 89. These pulses close switch 41 to permit R.F. current 90 degrees out of phase with the original R.F. to be applied to coil 52. This prevents spread of the magnetic moments. At the end of period t2 multivibrator 46 opens switch 42 to apply an antiphase R.F. signal to coil 52 for period t3 (Fig. 5) to produce a 90-degree counter nutation. After period t3 all switches 39, 41 and 43 are open and the field along the z axis reduces to its initial value. Thus any alternating field of frequency fo will produce no resonance although during periods Tg2, Tg4, &c., coil 52 is energized. The output of coil 64 is indicated on meter 68 after passing through amplifier 65, R.F. detector 50, audio amplifier 50 and phase-sensitive detector 66 receiving a reference signal from multivibrator 58. By averaging the detector output meter 68 will respond only to the resonant signal. However, to reduce the size of the signal induced in coil 64 by direct coupling between it and coil 52 balanced phase-sensitive detectors 69, 70 detect any signals in phase or 180 degrees out of phase and 90 degrees out of phase or 270 degrees out of phase with the output of oscillator 35. The outputs of the two detectors are D.C. voltages the polarity of which indicates whether the signal is 0 or 180 or 90 or 270 degrees respectively. These D.C. voltages are fed through filter 75<SP>1</SP>, 77<SP>1</SP> to remove variations at the frequency of the output of multivibrator 58 and used to control the gain of amplifiers 79, 78 which insert signals either in phase or 180 out or 90 or 270 degrees out in series with the output of coil 65. Thus signals at frequency #o are cancelled unless modulated at the frequency of the magnetic field. Instead of the original alternating field being applied for a single period of length t2 it may be applied as a series of pulses spaced from each other. The frequencies of the two alternating fields need not be the same but must be phase displaced by an amount corresponding to the spacing of the two coils by which they are applied. An arrangement is described with reference to Fig. 7 (not shown) in which a timing circuit controls a reactance tube which in turn controls an oscillator to change the frequency of the oscillation abruptly while maintaining phase or corresponding parts of the two waves unchanged. Thus if both frequencies are applied by the same unit a 90-degree phase shift is introduced, while if the coils are spaced by 90 degrees the shift is unnecessary. In a further embodiment the initial alternating field is used to nutate the macroscopic moment through the steady state angle which the moments would have assumed in the second alternating field. At the end of this period the field is removed and field applied perpendicularly to the unidirectional field and at a frequency substantially different from the resonance frequency. The signal field must be so positioned as to be parallel to the component of the moment which is parallel to the xy plane. Specifications 791,866 and 796,366 are referred to.